Polyamide resins are used as engineering resins in various fields. In recent years, polyamide resins are increasingly required to have high heat resistance especially for use as automotive parts or electrical/electronic parts. For example, automotive parts, in particular, those to be disposed around the engine, are more and more required to have heat resistance with the increasing density of parts in the engine room as a result of the progress of functions in motor vehicles. With respect to electrical/electronic parts, the density of parts in electrical/electronic products is increasing with the miniaturization of the products, and the level of required heat resistance is rising accordingly. Furthermore, with the progress of SMT (surface mount technology), electrical/electronic parts are required to have high soldering heat resistance.
As one measure for satisfying these requirements, a technique of blending a crystalline aliphatic polyamide resin with an aromatic polyamide resin has been investigated (see, for example, JP-A-61-188453 and JP-A-62-218445 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, the resin composition obtained by this technique has insufficient heat resistance for use in SMT in which a high-temperature solder is used or for use as parts to be disposed around the engine and required to have a heat resistance higher than 150.degree. C. Hence, the prior art composition has not been put to practical use.
Under these circumstances, a crystalline aromatic polyamide resin based on hexamethyleneterephthalamide has been put to practical use as a polyamide resin having high heat resistance, and is being used in various applications. Since the homopolymer of hexamethyleneterephthalamide has too high a melting point to be molded, the hexamethyleneterephthalamide-based polyamide resin generally used is one for which part of the terephthalic acid was replaced with an aliphatic dicarboxylic acid such as adipic acid (see, for example, JP-A-4-239571) or one in which .epsilon.-caprolactam or the like has been copolymerized (see, for example, JP-A-1-249857). Since these resins have high crystallinity, satisfactory suitability for short-cycle molding, and satisfactory in-mold flowability, they are extensively used as flame-retarded compositions in the field of electrical/electronic parts, mainly small parts. However, the resin obtained using an aliphatic dicarboxylic acid in combination with terephthalic acid has a lower glass transition temperature than the resin obtained using terephthalic acid as the only acid ingredient, and is hence unsuitable for use in applications where high rigidity is required at high temperatures.
Disclosed as a means for eliminating the above drawback is a copolyamide resin composition obtained by replacing part of the terephthalic acid with isophthalic acid. This composition has come to be increasingly used as a material having a high glass transition temperature in fields where high heat resistance is required (see, for example, JP-A-3-072564 and JP-A-3-201375). However, this composition containing isophthalic acid units has problems that its crystallinity decreases with the increasing proportion of isophthalic acid units and that the composition has a low solidification rate during molding and is inferior in in-mold flowability and suitability for short-cycle molding, although it can retain a high glass transition temperature. Namely, the above prior art composition not only has a problem of reduced productivity but has quality problems such as susceptibility to in-mold burning.